I wonder if autoscaling is needed at all if adding some caching in front. Pick one of Varnish, nginx, AWS Cloudfront or Cloudflare, and I’d not worry too much about autoscaling.
Setting a TTL as low as one second in Varnish will lead to request coalescing, meaning only a single request will hit the backend per URL per TTL expiry (1 s). If you’re flooded by traffic from a shared link, you’ll probably have most of the traffic at that single URL. Set the TTL below 30 seconds, and you’ll probably don’t have to think about cache invalidation at all for a typical Wordpress blog.
Not sure how Wordpress compares, but at $dayjob, the thing that is the most hassle to scale and the first to fail during high load, is the database. I don’t see how this setup solves that bit.